Power systems that use power converters or other variable voltage power sources and energy storage devices, such as capacitors or batteries, experience challenges when attempting to charge a energy storage device that already contains a charge that is either unknown or different than the power converter's output voltage. If connected haphazardly, a large and potentially damaging current surge may result. Several approaches in the prior art have attempted to eliminate the possibility of a dangerous current surge.
One method is to discharge the energy storage device to a zero voltage level and then connect the energy storage device to the power converter when the power converter has an output voltage of zero volts. However, this method generally results in a waste of energy since the energy storage device's charge is generally discharged without being applied to a load.
Alternatively, a resistor can be placed in parallel with the energy storage device. The resistor restricts the current traveling between the power converter and the energy storage device, thus avoiding a current surge. However, this approach has two shortcomings. First, it is time consuming to wait for the energy storage device to achieve the same voltage as the power converter. Second, the resistor, while controlling the current, generates heat as a byproduct of the current passing through it. Thus a system built with a resistor, while workable, also tends to waste power.